1. Field of the Invention
The present invention is directed to a low profile polarization diversity planar antenna for communicating polarized radiation in broad frequency band, particularly suited for a relay antenna or cellular antenna in a mobile telephone system.
2. Description of the Prior Art
In accordance with increasing demands for miniaturizing space diversity antenna for mobile telephone system, it has been proposed to give a combination of a notch antenna and a patch antenna, as disclosed in the paper "A Flat Energy Density Antenna System for Mobile Telephone", IEEE Transactions on Vehicular Technology, Vol. 40, No. 2, May 1991 by Hiroyuki Arai, Hideki Iwashita, Nasahiro Toki, and Naohisa Goto. The proposed antenna comprises a ground plane, a patch with notches, and a feed plate carrying microstrip lines. The patch has a feed point at its center and is shorted to the ground plane at portions spaced radially away from the center so that the patch is cooperative with the ground plane to constitute the patch antenna responsible for vertical polarization with respect to the ground plane. The microstrip lines of the feed plate include feed lines which are located in a directly opposed relation to the individual notches in the patch in order to feed the resulting notch antenna responsible for horizontal polarization with respect to the ground plane. However, in this composite antenna where the patch is commonly used as a radiator element for the notch and patch antennas, mutual coupling between the notch and patch antennas remains great so as to make it difficult to separate horizontal and vertical polarization effectively.